Concordia curve

Figure 5. Concordia diagram similar to Figure 4 illustrating the concordia curve for initial = 150 (appropriate for marine samples), with age in ka depicted parametrically along concordia. Also illustrated are continuous uranium gain/loss model curves for samples with primary ages of 80 ka (dashed) and 150 ka (thin solid curve). See text for discussion of this model and related models (after Cheng etal. 1998).

Figure 6. A portion of the concordia curve for initial =150 and four data points (Edwards et al. 1997) for coral samples from the last interglacial terrace in Barbados. The points lie along a hne, and illustrate the possibility of using best-fit lines through discordant points to extrapolate to concordia and possibly the tme age of a set of samples (after Cheng et al. 1998).

Table 3.4. Iterative calculation of the two age intercepts with the Concordia curve for a zircon linear array having a slope a = 0.256 and an intercept =0.094613.

Tera-Wasserburg diagram for lunar basalt 14053. The measured data define a discordia line that intersects the -axis at (207 /206 ), = 1.46. The slope of the line, based on two whole-rock samples and a magnetite separate, is -0.88366. The intersection of the discordia line with the concordia curve gives the age of the sample. After Faure and Mensing (2005). 

BOX 1.1 FIGURE 3 U-Pb Concordia curve showing the compositions of individual analyses as error ellipses. The data, which are for a single grain, show that the grain has a core which is about 3,350 Ma old and a rim which is about 2,700 Ma old. The rim has experienced subsequent lead loss, giving rise to a series of discordant points on the Concordia diagram. This lead loss reflects a later thermal event at about 1,000 Ma. 

Fig. 5.41 (a) Isoleucine epimerization in total hydrolysates from fossil foraminiferans Globomtalia tumida and Orbulina umversa in eastern Atlantic deep-sea sediment core 13519-2 (b) concordia curve based on (a) (A = D-alloisoleucine, I = L-isoleucine after Muller 1984). 

Fig. 3.17 The U-Pb concordia curve is plotted by solving Eqs. 3.16 and 3.17 for the same value of t in order to obtain the 206p /238 j 207p 3 /235u j- tios of points that represent concor-...

Different size fractions of the zircons extracted by Vocke and Hanson (1981) from the Vida Granite in DVDP6 define a chord that intersects the concordia curve at 447 34 Ma and at 0 Ma. The upper intercept is the crystallization age of zircon in the Vida Granite and the lower intercept (0 Ma) indicates that the zircons may have been losing radiogenic lead by continuous diffusion as a result of chemical weathering. 

These equations are used to calculate compatible sets of 2o pb p u and Pb / U ratios for selected values of t. The resulting values of these ratios are the coordinates of points that define the concordia curve which is the locus of all points whose coordinates yield concordant U-Pb dates. Any point that represents a mineral whose measured 206pjj /238u 207p j /235pj j-gbos do... 

Theoretically, initial Pb (also called common-Pb) is easily subtracted by measuring PhF Pb in the mineral. Then, the corrected data plotted in the (206pb/238u) =/(2o Pb/22 U) should be on the Concordia curve if the system has remained closed for U and Pb from the time of the formation of the mineral. Potentially, a meaningful age can be calculated for each spot analysis. 

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